ABSTRACT In the female rat, sexual receptivity is regulated by the sequential exposure of limbic- hypothalamic circuits to estradiol and progesterone. The goal of our research is to understand mechanisms through which progesterone acts to facilitate sexual receptivity. Estradiol stimulates a multisynaptic circuit to induce -endorphin (-END) release that activates -opioid receptors (MOP) in the medial preoptic nucleus (MPN), which inhibits sexual receptivity - lordosis. Subsequent progesterone treatment reduces MOP activation in MPN, presumably through the inhibition of -END activity in the hypothalamic arcuate nucleus (ARH), and facilitates lordosis. We have demonstrated that orphanin FQ (OFQ) in the ARH, acting through its cognate receptor, NOP, mimics progesterone by reducing MPN MOP activation and facilitating sexual receptivity. Our preliminary data suggest that progesterone acts through the OFQ-NOP neuropeptide system to relieve estradiol inhibition: i) OFQ neurons in ARH contain intracellular progesterone receptors, and ii) -END neurons express NOP that hyperpolarize - END when activated. These actions relieve MPN MOP activation and facilitate sexual receptivity. However, the mechanism(s) that progesterone uses to regulate this circuit is unclear. Progesterone may exert its effects through multiple progesterone receptors and signaling mechanisms. We propose studies to determine the progesterone signaling mechanisms that are involved using our well developed steroid responsive ARH-MPN lordosis circuit. We will test the hypothesis that progesterone uses multiple signaling mechanisms to facilitate lordosis by activating the OFQ-NOP system to inhibit estradiol-induced activation of -END neurons. We will examine both presynaptic and postsynaptic mechanisms through which estradiol and progesterone may be regulating OFQ neurotransmission in the ARH. The proposed experiments will also test whether: 1) NOP are expressed in -END neurons that project MPN; 2) progesterone inhibits -END neuronal activity through NOP; and 3) progesterone signals through multiple mechanisms to regulate the activity our model ARH-MPN opioid lordosis circuits. These experiments will use histological, electrophysiological and behavioral techniques to provide important fundamental information on progesterone signaling and regulation of neurotransmission in opioid circuits that regulate sexual receptivity.